The present invention relates to a welding robot for automatic welding of spirally bent tubes for producing dynamically balanced thrust nozzles comprising a welding head, a sensor arrangement which scans a course and a cross-sectional geometry including a gap width of a joint gap between components to be welded together, an evaluation unit coupled to the sensor unit and which generates setting signals, and a setting device coupled to the evaluation unit and the welding head, which setting device is controlled by the setting signals and adjustably sets welding parameters and guides the welding head along the joint gap in a multi-axially adjustable manner.
Welding robots with a laser-optical sensor device by which the welding head is adaptively guided along the spatial path of the joint gap, where the welding head control is automatically changed at the start or end of the seam for the purpose of providing overlapping or multiple welds are known. However, such welding robots which operate purely with position control and a rigid repetition of a welding process course once it has been set, without being able to compensate for unavoidable interferences in the welding process, produce a very irregular welding bead formation when welding components with an uneven geometry of the joint gap. This is particularly true for spirally bundled, thin-walled tubes, for example for regeneratively-cooled thrust nozzles. There are also frequent welding errors with respect to the effective seam length. The cause of this lies in too small or too large local melt bath sizes in the area of the component edges.
With the known automatic welding robots of the initially-mentioned type, the welding process itself is affected in addition to the kinematic welding control for reasons of an improved welding quality. In this way not only the path of the joint gap, but also the geometry of the joint gap or the weld seam geometry are continuously scanned during the welding operation in order to adapt the definitive welding parameters, such as the welding path speed or the welding current, to the local welding requirements. Thus the characteristic seam data, such as the weld seam width, the seam camber or the penetration can be maintained at the predetermined set values, independently of the occurring interferences. In the process, the translation of the values measured by the sensor into corresponding welding parameter set commands takes place computer-assisted with the aid of regulating or control elements on the basis of control algorithms, non-linear observer models or multi-dimensional performance characteristics data, amended in accordance with the sensor signals. Because of the complexity of the welding process, this entails such a large structural and computing effort that in actuality, along with having to accept a reduced welding quality, the welding process control is limited to one or at best a few variable welding parameters, such as the welding current and/or the wire feed rate. For reasons of a real-time signal evaluation it is mostly necessary to keep the welding path speed clearly below the maximum value which would be possible per se on the basis of welding technology.
An object of the present invention is to provide a welding robot of the initially-described type which achieves a high welding quality with a small apparatus and computer expenditure and with full use of the maximal welding path speed possible on the basis of welding technology, even under difficult welding conditions and particularly in connection with a very irregular joint gap geometry.
This and other objects are achieved by the present invention which provides a welding robot particularly for automatic welding of spirally bent tubes for producing dynamically balanced thrust nozzles comprising a welding head, a sensor arrangement which scans a course and a cross-sectional geometry including a gap width of a joint gap between components to be welded together, an evaluation unit coupled to the sensor unit and which generates setting signals, and a setting device coupled to the evaluation unit and the welding head, which setting device is controlled by the setting signals and adjustably sets welding parameters and guides the welding head along the joint gap in a multi-axially adjustable manner. The evaluation unit includes a data memory with a plurality of welding parameter data sets respectively associated with different joint gap cross section geometries, and a control stage which selects the respective welding parameter data set controlling the setting device during the welding process in accordance with the detected joint gap geometry.
The component scanning by the welding robot in accordance with the invention is limited to a few measuring signals identifying the joint gap geometry, including the joint gap width. These measuring signals are translated into corresponding welding process control commands by a data call-up which uses simple computer technology without complicated, time-consuming computing operations. By means of this it is possible to assure an automatic, sensitive real-time adaptation of all important welding parameters, and therefore a high welding quality, during the ongoing welding process, without it being necessary, because of a time delay in signal evaluation, to maintain the welding path speed at a low value which per se is unnecessary for assuring quality.
In certain preferred embodiments, the edge shape of the joint gap is scanned in addition to the gap width and is incorporated into the welding parameter control in such a way that, when a contour change of the component edges is sensed, in the interest of an increased quality assurance the automatic welding process is either interrupted or is continued by calling up an appropriately changed welding parameter data set, e.g. with a changed electrode distance and/or a different welding current intensity. For reasons of an improved quality assurance, a level difference of the component edges is compensated by a corresponding reorientation of the welding electrode in a practical way in certain embodiments of the invention. This prevents an asymmetric distribution of the arc with respect to the component edges.
For reasons of a structurally simple and sturdy design, along with simultaneous high measuring accuracy, the joint gap is preferably scanned in certain embodiments, in an optical manner. This is done by a laser beam strip directed crosswise to the joint gap. A measurement value sensor which detects the cross-sectional data of the joint gap is provided for signal evaluation.
In accordance with certain embodiments of the invention, the welding parameter data sets which are experimentally determined on sample workpieces prior to starting the automatic welding process respectively contain adjustment commands not only for the welding current and the welding path speed, but preferably also for the electrode distance and a supplemental material supply, depending on the joint gap, for protective gas shielded arc welding. Certain embodiments of the present invention additionally scan the degree of reflection in the area of the joint gap edges by means of the sensor arrangement to include a protective gas adjustment command called up in accordance with the detected degree of reflection in the welding parameter data sets.
In certain embodiments of the invention, a video camera is attached to the welding head for visual control of the weld seam during the ongoing welding process.
In particularly preferred embodiments of the invention, a control device reacting to an upper and a lower welding voltage threshold value is disposed in the current supply circuit, by which a frequent source of welding errors, the cause of which is found to be excessive electrode consumption or spatters of material adhering to the electrode tip and consequently inadmissible welding voltage values, is effectively removed in a structurally simple manner.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.